1. Field of the Invention
The present invention relates to a method for fabricating a mask read-only-memory (mask ROM), and more particularly to a method for fabricating a mask ROM with diode cells.
2. Description of the Prior Art
Memory cells of a mask ROM (mask read-only-memory) are is generally made using channel transistors. Programming of the ROM is achieved by selectively implanting ions into the channels of these transistors. By implanting ions into the channel regions of specified transistors, threshold voltage of the devices changes. Hence, the xe2x80x9conxe2x80x9d or xe2x80x9coff xe2x80x9d state of the memory cell is coded. A mask ROM cell is formed by laying a polysilicon word line (WL) over a bit line (BL), and the memory cell channel is formed in the region underneath the word line between neighboring bit lines. Normally, each ROM cell is in a logic state of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d depending on whether ions are implanted into the channel region or not. The advantage of being able to program the state of each ROM cell by an ion implant operation is that semi-finished ROM products can be made. Once the required program codes arrive, a mask can be made and then the final ion implant operation can be carried out, thereby shortening customers"" delivery date. However, the method requires the production of one more photomask to carry out an ion implant operation. Moreover, reliability of the final ROM product is very much dependent upon the quality of the ion implant operation.
FIG. 1A is a schematic top view of a conventional mask ROM. Referring to FIG. 1B, a gate oxide layer 102 is formed in a p-type substrate 100. Gates 104 are formed over the substrate 100 as word lines. An ion implantation process is performed by using gates 104 as masks to form n-type source/drain regions 106 as bit lines. The bit lines cross the word lines perpendicularly. Channels are formed below the word lines. The state of each memory cell is determined by the channels. The method of closing the channels is to implant p-type ions into specific channels 107 to form code ion implant regions 110.
The conventional method for fabricating the mask ROM is shown in FIGS. 1B and 1C. Referring to FIG. 1B, a p-type substrate 100 is provided. An isolation region 101 is formed in the substrate 100. A gate oxide layer 102 and gates 104 are formed in sequence on the substrate 100. An ion implantation process is performed by using n-type ions as dopants to form a plurality of source/drain regions 106 in the substrate 100. Channels 107 are formed between two adjacent source/drain regions 106.
Referring to FIG. 1C, a patterned photoresist layer 108 is formed over the substrate 100 to expose subsequently formed code ion implant regions. An ion implantation process is performed by using phosphrous-31 as a dopant. The implantation energy is 160 KeV and the dosage of the dopant is about 1xc3x971014 ions/cm2. After performing an annealing process at 850xc2x0 C., code ion implant regions 110 are formed. The photoresist layer 108 is removed. Thus, an encoding process of the mask ROM is completed.
The conventional mask ROM device uses a channel transistor as a memory cell. The implantation of ions into the channel determines the binary data xe2x80x9c0 xe2x80x9d or xe2x80x9c1xe2x80x9d. However, when the mask ROM device is shrunk down, the issue of the code pattern resolution is enhanced and the code ion implanted region cannot be well defined.
Accordingly, it is an intention to provide an improved method for fabricating a mask ROM, which can overcome the above drawback encountered when the mask ROM device is increasingly shrunk down.
It is one objective of the present invention to provide a method for fabricating a mask read-only-memory with diode cells, which forms a structure of a contact plug/a PN diode instead of a channel transistor as a memory cell. The process for forming the structure of the contact plug/PN diode is simple and can provide a well-defined code area even though the device dimension is shrunk down.
It is another objective of the present invention to provide a method for fabricating a mask read-only-memory with diode cells, since the area of the substrate occupied by one PN diode cell is smaller than that of a channel transistor, a high-density mask ROM device can be obtained by the present method.
It is a further objective of the present invention to provide a method for fabricating a mask read-only-memory with diode cells, which provides a simple process to form a structure of a contact plug/a PN diode instead of a channel transistor as a memory cell. Therefore, the manufacturing cost can be reduced.
In order to achieve the above objectives, the present invention provides a method for fabricating a mask read-only-memory with diode cells. A semiconductor substrate is provided and a buried diffusion layer with a first conductivity is formed in the top portion of the semiconductor substrate. A plurality of shallow trench isolation regions are formed in the semiconductor substrate, and making the buried diffusion layer to a plurality of bit lines. An interlayer dielectric layer is formed over the buried diffusion layer and the shallow trench isolation regions. A photoresist layer with a mask read-only-memory code pattern is formed on the interlayer dielectric layer. Performing an anisotropic etching process to form openings in the interlayer dielectric layer unto the exposed regions of the buried diffusion layer using the photoresist layer as an etching mask. Then, the photoresist layer is removed. Performing ion implantation to form a diffusion region, with a second conductivity opposite to the first conductivity, in each of the exposed regions of the buried diffusion layer. Thereafter, a contact plug is formed in each opening unto the diffusion region. A conductive layer is formed on the interlayer dielectric layer for serving as word lines. As a consequence, a structure of a contact plug/a diode cell is obtained, which is used as a memory cell of a mask read-only-memory (mask ROM) instead of a channel transistor. The area of the semiconductor substrate occupied by one diode cell is smaller than that of the channel transistor. Thus, a high-density mask ROM device can be obtained by the present method. Furthermore, the process for forming the structure of the contact plug/diode cell is simple and can provide a well-defined code area even through the device dimension is shrunk down. The manufacturing cost is also reduced.